Magnetoencephalography (“MEG”) involves detecting magnetic fields produced within a subject's brain. Information about such biomagnetic fields is most useful when it can be associated with particular structures within the subject's brain. One can localize the subject's head relative to the measured magnetic fields. To do this, the position of the subject's head relative to the magnetic detectors used to detect the magnetic fields must be known.
One way to localize a subject's head is to attach small coils at three or more known locations on the subject's head. The coils create fluctuating magnetic fields when alternating electrical currents are passed through them. Magnetic detectors are used to detect the coils' magnetic fields. The location of the coils, and thus the location of the subject's head, can be determined from the detected magnetic fields of the coils.
One problem with some current head localization systems is that the coils generate large magnetic fields that can saturate the very sensitive magnetic field detectors used to detect biomagnetic signals. Because of this, head localization cannot be performed while biomagnetic signals are being measured. With such current systems it is necessary to perform head localization before and/or after acquiring data about the biomagnetic signals. Such systems assume that the position of the subject's head changes predictably and can be determined from its positions before and/or after the biomagnetic signals are measured. These assumptions are not always accurate.
Acquiring biomagnetic signals is often performed over significant time spans. During these times a subject's head is likely to move even if the subject is attempting to stay still.
Magnetic noise can exacerbate the problems with head localization. The magnetic detectors used to detect the magnetic fields from the coils will also pick up noise signals, such as signals at the power line frequency (generally 60 Hz in North America and 50 Hz in Europe) and at harmonics of the powerline frequency. Such noise signals can degrade the accuracy with which the positions of each of the coils can be determined. Magnetic noise includes environmental background noise, noise resulting from operation of the magnetic detectors and other components in the signals from the magnetic detectors which do not arise from the magnetic fields of the coils.
There is a need for systems which permit continuous localization of heads and other structures during the acquisition of biomagnetic signals. There is a particular need for such systems which are relatively insensitive to noise.